Pressure transmitter



Feb 23, 1943- c. H. THOMPSON ETAL PREISSURE TRANSMITTER Filed Feb. '12, 1941 4 l f l r l r l r a r f r f r r n r r a l r r l f 4 f r l n r '@llll mi 301 4 Sheets-Sheet 1 Inde/Lions' CarasIT/zongosoa and Colemmzfoorfe,

Feb. 23, 1943. C; H, THMP'SONETAL 2,312,201

' PRESSURE TRANSMITTER.

Filed Feb. l2, 1941 4 Sheets-Sheet 2 orngy Feb.- 23, 1943.A v c, H. THOMPSON ET AL 2,312,201

PRESSURE TRANSMITTER Filed Feb. l2, 1941 4 Sheets-Sheet 5 Feb- 23, 19j3- c. H. THOMPSON ETAL 2,312,201

PRESSURE TRANSMITTER Filed Feb. l2, 1941 4 Sheets-Sheet 4 atented Feb. 23, 1943l rrr-:fo s'rA'ra s *raArENT- 'or-fries j amazon PRESSURE SMITTER i Uharles H. Thompson; Huntingdon Valley, and

Coleman B. Moore, lCarroll Park, Pa.

application February r2, 1941, serial No. stacco ro claims. (cl. isi- 153i This invention relates to pressure' transmission units, and more particularly to a pressure trans- Y mission unit of the pneumatic type.

' lt is the principal object of the present inventionA to provide an improved .form of a pressure transmission unit suitable for use for purposes ci indication, recording vor control..

lt is a further object of the present invention to provide a dierential pressure responsive unit which will havev a wide range of usefulness in its l various applications.

1t is a further object of the present invention to provide a unit responsive to pressure dlierentiais with provisions for rebalancing.

It is a further object of the present invention to provide a dierential pressure responsive unit with provisions for multiplying the resultant ofv the dierential pressure.

it is a further object of the present invention to provide a pressure transmission unit in which the total motion of the movable parts, after rebalancing, is reduced to a minimum so that the linear characteristics of the transmitted pressure are func-.nested by the structure. 1.. @ther objects of the invention will be apparent from the annexed specication and claims.`

The nature andcharacteristic features of the invention `will be more readily understood from the following description,v takenl in connection with the accompanying drawings forming part hereoi", in which:

ent invention-for iiuid 'flow indication, recording or control. l

` It will, ofcourse, be understood that the description and drawings herein are illustrative vmerely, and that various modifications and changes may be made in .the structure disclosed without departing from the spirit of the invention. Referring more particularly to Figs. 1 and 2 of the drawings,' the embodiment of the pneu-1 matic differential pressure responsive unit is there illustrated generally at it). The unit it prefer-4 'ably includes an external side wall portion ii,

an upper supporting end wall portion i2 and a lower supporting end wall portion i4, the supporting end wall portions i2 and il being secured to the side wall portion il at the ends thereof with fluid-tight joints and the whole being made of suitable strength to withstand the pressures to-which the `unit is subjectedin use. While thev unit it is shown as located in a particular position, it is not limited in its operation to this.

. position.

Figure l. is a vertical central sectional view oi a preferred embodiment of a pneumatic ldierential pressure responsive transmission unit constructed in accordance with the present invention and provided with a pilot valve as an integral part thereof;

Fig. 2 is a horizontal sectional view taken approximately on the line 2-2 of lFig. 1;

Fig. 3 is a vertical central sectional view of another preferred embodiment of the .pneumatic 'of the present invention for use where a pilot valve is not required, parts being shown in elevation;

Fig. 4 is a. horizontal sectional vlew taken ap- Fig. v5 is a diagrammatic view illustrating one mode of use of the pneumatic unit of the present- Vproximately on the line ,4 4 of Fig. 3;

l invention;

Fig. 6 is a diagrammatic viewT illustrating the use of the pneumatic unit of the present invention for liquidlevel indication, recording or'control;

Fig. 7 is a diagrammatic view illustrating the manner of use of the pneumatic unit of the present invention for specific gravity indication, recording or control; and

Fig. 8 is a diagrammatic view illustrating the .manner ot use of the pneumatic unit of the presdierential pressure responsiveitransmissicn unit ably mounted in the end wall portion i2 and ei'rtends to but is spaced a veryk short predeter- Intermediate the supporting end wall portions l i2 and it a fiat metallic bellows end plate member it is provided and has secured thereto at the 'outer edge in uid-tight relationship a flexible or extensible metallic Abellows i6 which is connected to the bellows plate member I5 and extends therefrom tothe supporting end wall portion i2 and is secured to the end wall portion I2 in iluid- Y tight relationship.

action of fluid under pressure within the cham-H ber it.

The' end wall portion i2 has mounted therein a. nozzle member i9 which is preferably adJuSifmined distance from the bellows plate member i5.

The nozzle member i9 has an interiorbody por tion 23 which is in communication with the chamber 35 Within the bellows il through a discharge port 2t. The quantity-,of fluid passing through the discharge port 2d will be determined by the location, for a given set of pressure conditions, of the bellowsplate member i5 with respect to the discharge port 2l, as hereinafter more fully set forth. A passageway 211s provided in the .end wall I2 for venting the interior of the chamber 36 within the bellows i1 to the atmosphere and so that the pressure within the chamber 88 will be atmospheric.

A metallic bellows 25 is also provided, of the same diameter as the metallic bellows I1, and is secured to the bellows plate member I5 in fluidtight relationship. The bellows 25 extends from the plate I5 to the end wall I4 to which it is likewise secured in fluid-tight relationship.

The space within the side wall Il and between the end walls I2 and I4 and outside the bellows` I6 and the bellows 25 provides a fluid pressure l pressure of this fluid will .be referred to as al lower pressure and identified as pz.

A duid connection 88 is also provided for introducing fluid under pressure'to the chamber I8. 'Ihis iluid connection preferably includes passageways 84 and 85 provided in the end portion I2. This fluid may also be a liquid or a gas, in accordance with the use which is made of the pneumatic unit. The pressure of this fluid will be referred to as a higher preure and identied as p1.

The upper end wall portion I2 is preferably also provided with a plurality of vents 81 and 88 in communication respectively with the chambers I8 and 28, the vents being closed by suitable screw plugs 88 and 48 and providing for initially filling the chambers I8 and 26 with liquid when liquid is employed in the instrument.

-The space within the bellows provides a fluid pressure chamber 4| and an effective area an isA available on the bellows plate I5 in accordance with the size of the bellows 25 and I1 for the action of fluid under pressure within the chamber 4I.

The eil'ective areas aiA and a2 on the bellows plate4` I5 are preferably proportioned, by the selection of bellowsfof suitable size and with the desired internal and external diameters, to provide the desired ratio of amplication of the pressure diil'erential which is effective on th bellows plate I5. For example, if the eifective areas arand aa are in the ratio of 20 to 1 an amplification of 20 to 1 of the pressure diiferential will be obtainedA as hereinafter referred to.

The end wall portion I4 preferably has mounted therein an overload stop member 28 which extends longitudinally within the bellows '2 5 and is terminated a short distance from the bellows plate member I5 for limiting themovement thereof..

The stop member 28 is preferably in the form of a tube and the interior thereof has a supply or pressure equalizing passageway 28 extending therefrom to the pilot valve mechanism to provide for communication therewith, as hereinafter more fully pointed out.

'of the supply, which may be selected to suit the particular requirements, will be referred to as ps.

Any desired pilot valve mechanism may be employed in connection with the instrument. The pilot valve mechanismhereinafter set forth is measure of diiferential pressure in terms of duid pressure for utilization at a remote location. The pilot valve mechanism may be provided within vand having a movable end wall 88. A casing 8l,

also mounted. in fluid-tight relationship on the end wall portion I4, encloses the bellows to provide a fluid pressure chamber 88 outside the b'ellows 85. The chamber 88 is supplied with air from the supplycondult 48 through a passageway 88 and a passageway 80. A flow control member 58, shown in more detail in Fig. v4, is mounted in theend wall portion for restricting and controlling the quantity and pressure of" fluid supplied from the supply conduit and through the passageways 88 and 88 to the chamber 88.

'I'he iiow control member 50 has a constriction or oritlce 5I, and the constriction or orifice 5I is preferably of smaller size than the discharge port 24., to permit of a drop .in the pressure beyond the orifice 5I of the duid supplied from the source.

The chamber 88 is in communication with the nozzle member I8 through a connector 8| mounted in the wall of the casing 81, a conduit 82 extending to the end member I2 and a passagehas a pilot valve actuating rod 84 connected thereto for actuating the pilot valve 85 upon a change in the pressure in the chamber 88.

The pilot valve mechanism includes a pilot valve chamber 86 which hasfa fixed seat 81 at one end thereof and has a removable seat member 88 at the other end thereof. The removable valve seat member 88 has a fluid passageway and guide portion 88 therethrough and provides for communication betweem the. supply conduit 46 and the pilot valve chamber 88 and supplying of fluid to the pilot valve chamber 86 in accordance with the positioning of thev pilot valve 85. A passageway I8I extends from the valve chamber 88 to the space |82 adjacent the bellows 85 for the controlled discharge of fluid from the pilot .valve chamber 88 ,in accordance with the positioning of the pilot valve 85. An

. exhaust port |88 is provided in communication by the valve rod 84. has spaced `seating portions |85 and |88. The seating portion |85 of the pilot valve 85 is adapted to be moved to the proper position with respect to the valve seat 88 to provide the'desired control bylthrottling or cutting` off thefiow through the passageway in accordance with the positioning of the valve 85. The

valve 85 is guided and supported bythe move- -ment of the end portion I81 in the guide por-` tion 88. The seating portion |88 of the valve 85 is, at the same time, moved to the proper position with respect to the valve seat portion y81 to provide for the controlled and throttled suited for the purpose of transmitting the desired 78 pilot valve which controls the ilow discharge of fluid through the passageway III. The pressure prevailing in the valve/)chamber 88 will be determined by the positioning of the i into and out ofthat chamber. l

An instrument connection 58 isprovided and erably air, being connected thereto.

. p asiaaoi is in communicationwith the pilot valve chamber 96 through the passageway ii'i and addi-u tionally with the chamber di through'the passageway 29 which extends from the pilot valve chamber 96 4to the chamber di.,

A iiuid pressure responsive indicating, recording or control mechanism M is connected tc the instrument connection 55, and the form of the invention shown in Figs. l and 2 is particularly static pressure and will be the pressure utilized for rebalancing 'and prevailing in the chamber di This pressure will be referred to as the metering pressure and identied as p4. v

The form of the invention illustrated in Figs. 3 and 4 is similar to that illustrated in Figs. l and 2 and includes the provision of the side wall. portion il, the end wall portions i2 and Id, the bellows i6 andill', the bellows plate' member i and the bellows 25. The iuid connection d@ is provided for introducing fluid under pressure l to the chamber' 2li and includes' passageways 3i and 32 in the end member it. The iiuid con-.- nection 33 is also provided for introducing uid under pressure to the chamber it and includes passageway/s 3d and dit provided in the end p0rtion i2.

The end wall portieril i2 preferably has mounted therein a removable overload stop member 2@ which extendslongitudinally within the bellows il and is `terminated a4 short distance from Athe bellows plate member i5 for limiting the movement thereof. Thestop' member 2li has 4a' central opening 2i therethrough in communication with an opening 22 in the end wall portion i2 to provide for communication of the chamber te in the interior o the bellows il with the atmosphere and so that the pressure within the chamber 36 will be atmospheric. t

A nozzle member it is provided within the chamber di and is preferably adjustably mounted in the end wall portion ifi and extends tol but is spaced a very short predetermined distance from the bellows plate i5 and within the chamber di. The nozzle member d3 has a discharge port it which is in communication -with the atmosphere through the open interior body d5 responsive indicating, recording or control mechanism Ivi-is directly connected to the instrument connection 55 in' accordance with .the speciiic vuse which is made of the pneumatic unit. li indication or recording is desired, the mechav 'nism may take the form of an indicating or recording instrument and if control'is desired, this mechanism may takethe form of a pressure responsive or pressure controlled mechanism.

The pressure transmitted through the instrument connection 55- will, upon balancing, bea

static pressureand will be the pressure prevailing in the chamber di. This metering pressure will be referred to 'and identified as p4.

The operation of the pneumatic unit will'now be pointed out, attention rst being given to the features onnmonV to the diierentiormsoi the invention herein illustrated. The pressure pi will be available through the conduit ttf and in` the chamber it and upon the effective area. a1

of therplate member i5 in one direction. The pressure p will be available through the conduit il@ and in the chamber 2B and upon the same leectivfe areaai of the plate member i5 in the oppositedirection. Fluid under pressure is sup-U plied, inthe manner hereinafter pointed out; to the chamber ii and upon the effective area a2 of the plate member i5 in the same direction as the pressure p2. The pressure of the nuid vin the chamber il is' utilized for rebalancing. The iiuidin the chamber il will have the eiect on -the portion of the plate member i5 on which this pressure fluid is eiective, l. e., the effective area ci, of counterbalancing the dierenc'e between..

the pressures p1 and p2 exerted on the end plate member i5 by the and 2t. I

Upon an vincrease in the dderential of the pressures p1 land p2 there will be a tendency to compress the bellows 25. and extend the bellows le and il by reason of the preponderance of the fonce exerted by the fluid under the pressure pi.

.position so asfto changethe permissible ow of air through the discharge ports 2d or d@ of the nozzles i9 or d3. As hereinafter pointed out, an

increase in thepressure in the chamber iii is f eected and this moves the plate member i5 tov a new equilibrium position so theta new balancing or metering pressure is edective within the chamber di, and this pressure is 'available at the instrument connection 55 for transmission.

The force eectivein the chamber ,di and .acting on the plate member i5 will be the product oiv the metering pressure p4 and the eective ci the nozzle member d3. The quantity of uid I passing through the .dischargev port 'Mi will be determined by the location, for a given set of pressure conditions, of the bellows plate i5 with respect to the discharge port dii, as hereinafter area upon which this pressure acts.A Fromvthis it will be seen that the rebalancing is proportionateA to the respective sizes oi the areas aiiand aa and byvsuitably proportioning. the arcani with respect to the arealai/the metering pressure p4 and thel changes thereof will be multiples of the di'erent'ial pressure (prepa) and the changes thereof s o that a. wide range of amplification is available independently of the use lof the pilot valvemechanism. f

, Upon a'decrease in thejdiiferential pressure the plate member i5 will be initially impelled in the opposite direction fandfso as to compress the bellows le and si and extend the benows 25 and again vary the permissible iiowof air through the discharge port 24 or d6 of the nozzle it or. at. *The pressure in .the chamber 4l will be caused to decrease to a new equilibrium and the` iluids in the chambers it plate member I will take a new equilibrium position in accordance with the resultant of the pressures acting-thereupon. The pressure p4 -in the chamber 4I will be available for transmissio as hereinafter pointed out. The control of the supply of the fluid to the chamber 4I for rebalancing diiers somewhat in the twoiorms of the invention.

In the form of the invention illustrated in Figs. 1 and 2, the control of the pressure of the iluid supplied to vthe chamber 4I is effected through the control of the pilot valve 95. Fluid is supplied at the supply connection 48 and a porj tion passes through the passageway 89, through the control member 58 with its constriction or orice 5|, and through the passageway 90 to the chamber 88 where it is eective on the bellows 85 in that chamber. The iluid also passes from the chamber 88 through the connector 9i, the

conduit 82, the passageway 83 in the end member I2 and through the interior portion 28 of the nozzle member I9 to the discharge port 24. 'The iiow through the discharge port 24 is determined by the positioning, for a given set of diiferential pressure conditions, of the plate member I5 with respect to the discharge port 24. It the flow through the discharge port 24 is reduced, the pressure back through the nozzle I9, and its connections to the chamber 88, and in the chamber 88 will increase andthe force thereof will be eii'ective to compress the bellows 85. Pres.

sure fluid will also be supplied to the valve chamber 85 through the passageway 98 in the valve seat member 98, in accordance -with the positioning of the pilot -valve 95, and will be discharged through the passageway IUI and through the space |82 and the exhaust passageway |03 to the atmosphere, also in accordance with the positioning of the pilot valve 95.

The pressure of the iluid in the pilot valve chamber 96 will be effective through the passageway 29 and in the chamber 4I and will be available against the effective area a2 on the plate member I5.

Upon an increase inthe differential pressure, the bellows plate I5 will tend to move downwardly. This will permit more iiuid to escape through the discharge port 24 and will cause a drop in pressure which will be effective within the bellows chamber 88. Upon a drop in the pressure within the chamber 88, the bellows 85 will expand and the pilot valve 95 will be actuated towards the right as illustrated. 'I'his movement of the pilot valve A85 will admit air at increased pressure to the pilot valve chamber 93 and therefrom to the'chamber 4I and decrease the discharge of air from the pilot valve chamber 95. The increase in pressure in the chamber 4I will be eilective for moving the bellows plate member I5 with respect to the discharge port 24 until equilibrium pressure conditions are attained. 'The bellows plate I5I will thus be returned to a new rebalancing position.

. Upon a decrease of the differential pressure, the bellows plate I5 will tend to move upwardly and this will decrease the discharge of air through the discharge port 24 and will cause an increase in the pressure within the chamber 88. The pilot valve 85 will be actuated to throttle the air entering through the passageway 99 and increase the discharge from the pilot valve chamber 85 so'that a reduced pressure will be eiective for rebalancing in the chamber 4I and a new set of equilibrium -conditions will be attained.

rebalancing will be available at the instrument connection 55 for indication, recording or control,

as desired.

In the form of the invention illustrated in Figs.

' 3 and 4, upon an increase in the diierential of the pressures p1 and pz, the bellows plate member I5 will be initially moved to a position so as to reduce the permissible flow of fluid through .the discharge'port 44, or even to a position to close 0E the escape of air through the discharge port 44. 'I'he pressure in the chamber 4I will increase and move the bellows plate member I5 to a new equilibrium position so that a new metering pressure is effective within the chamber 4I. The venting of the chamber 4I will continue. This metering -pressure will be transmitted through the passageways 48 and 48 and the instrument connection 55 to the indicating, recording or control mechanism.

Upon a decrease in the differential pressure, the bellows plate member I 5 will be initially moved in a direction awayfrom the nozzle 43 and increase the permissible iiow of air through the discharge port 44. The pressure in the chamber 4I will decrease to a new equilibrium and the bellows plate member l5 will take a new equilibrium position in accordance with the resultant of the pressures acting thereupon. The pressure p4 in the chamber 4I will be transmitted through the passageways 49 and 48 and the instrument connection 55 to the indicating, lrecording or control mechanism as before.

The rebalancing by the use of the air pressure in the chamber 4I permits of attaining equilibrium conditions with a very small total range of movement of the bellows plate member I5 and of the order of afew thousandths oi'an inch, and this may be accomplished with a very wide range of pressure diierentials Since the bellows plate member I5 is responsive to very slight changes in pressure, and since by the rebalancing the bellows plate member is returned substantially to its initial position,vany non-linear effect of the bellows is rendered negligible.

From the foregoing it will -be clear that with the pneumatic unit of the present invention diierential pressures, as a medium for ascertaining changes of condition, may be translated and transmitted in terms of a variable transmitted static pressure of a iluid, amplied as desired', and this transmitted variable fluid pressure is available either directly or through asuitable vpilot valve for purposes of indication, recording or control. Speciiic instances of installations in which the penumatic unit may be employed will now be set forth. A'

In Fig. 5, the pneumatic unit I0 is illustrated in one mode of use for'liquid level indication, recording or control. A container or tank Ti is provided in which two liquids vI1 and I2 of diierent speciiic gravities are provided, the liquids being in face to face contact. The conduit 30 is connected to the upper portion ofv the container T1 and above the contact faces and the conduit v33 to the lower portion and below the contact faces of the liquids. The conduits 38 and 33 are respectively connected to the pneumatic unit. The conduit may have a liquid seal 60 and may also have a valve 8l for shutting oii communication, if desired.` The conduit 33 may also have a valve 52 therein for shutting oil.' communication. A cross connection 33 with a valve 54 therein may be provided between the conduits 30 and 33, for pressure equalization. A source S of pressure regulated and -iiltered air is connected to the eon asiaaoi 4 duit 48, and the instrument connection B5 may be connected to a suitable mechanism M for indifcat'ion or recording. The mechanism M may, in

turn, be connected for transmitting uid under pressure from a-source S', and controlled by the mechanism M, to a control valve 66 for regulating through these conduits and upon the end plate member l5 of the pneumatic unit it. The differential pressure will vary in accordance with the variation of the face to face level of the liquids il andl la. The successive dierential pressures will be translated by the pneumatic unit @t and transmitted as changes in the metering pressure of the uid which may be utilized forthe desired indication, recording or control of the face to face liquid level. i

In Fig. 6, the unit it is shown connected for liquid level indication, recording or control. The conduit 3U is connected to the tank T2 in which the liquid i is contained, above the level oi thev liquid, and may be connected through a liquid seal t@ to the pneumatic unit it. The tank is also connected, below the level of the liquid, by a conl duit 83, which may have a liquid seal d@ therein, to the pneumatic unit it. Theunit' l@ also has connected thereto the source S of pressure regulated and iiltered air and the mechanism M for indication, recording or control as desired. Upon a change of the level of the liquid in the tank T2, the pressures will be transmitted through the conduits to the end plate member l5 and the differential pressure eective on the end plate member l5 will varyin accordance with the level of the liquid in the tank T2. As the level changes the successive differential pressure changes will be translated Vby the pneumatic unit' iii and transmitted as ampled changes in the metering pressure p4, which pressure may be utilized at the vmechanism M for the desired indication, recording or control of the liquid level inthe tank Ta.

In Fig. 7, the pneumatic unit it is shownA as 4connected for the determination oi the specific vertically vspaced locations within the liquid Is.

For the purpose of ascertaining the pressure at one point in the liquid,y a bubbling tube 1u is pro 4 vided,- which extends downwardly through the liquid in the tank T3. Gaseous fluid, such as air,

'under pressure is supplied from a source S1l bling bottle 'l1 ilows through the bubbling tube 16, is a measure oi' the pressure at the lower end of the bubbling tube 18. The dierential of the pressures in the conduits 33 and et may thus be translated by the pneumatic unit it and the transmitted pressure will serve as a measure of the specific gravity or density of the liquid in the tank. j This measure of the specific gravity or density may then be indicatedecorded or controlled as desired by the mechanism M.

.The pneumatic unit I ll may also be utilized for l ilow control or iiow indication as illustrated in Fig. 8.' A conduit 80 through which the fluid to be measured is owing is provided with any desired type of oriiice 3l, the conduits 33 and 3B being respectively connected to the up-stream and down-stream side of the orifice di. -The v unit it also has the source or supply S of presthe pneumatic unit it of the present invention j for indication, recording or controi by the es, i

through apipe 1i, preferably having an adjust#- able restriction or valve l2 vtherein for controlling the ow,`and through a bubbling bottle 13 and the gaseous uid from thef ubbling bottle 13 is permitted to bubble through e lower end of thetube 10. The pressure transmitted through the conduit 33 is -thus a measure ci the :pressure at the lower end of the tube\'|9. Another bubbling tube 1i is also provided terminatingat a 1 different level within theliquid in and above the lower end of the tube 1t.` Gaseous fluid, such'as air, under pressure, is supplied from a sources:

` thr'ough a conduit lli which preferably has an adjustable restriction or oriflce 16 therein for controlling theflow and through a bubbling bot-y tle 11.- l The .pressure transmitted through the sure regulated and filtered air connected thereto by the conduit 48. The dierential pressure may thus be translated and transmitted with mechanism M.

We claim: 1. A iuid pressure responsive mechanism including an enclosing casing, a movable plate member within said casing and connected thereto by an extensible wall, said movable member having a iiat pressure control surface, means for applying iiuid pressure against one face of said v movable member and inside said extensible wall, means for applying iiuid pressure against a corresponding area on the other face oi said movable member and outside said extensible wall,

- and balancing means for applying' a balancing uicl pressure against another face portion of said movable member for balancing the dierence of the pressure applied by said other means, said Vbalancing means including a balancing chamber bounded in part by said movable member, a pilot valve for controlling the supplying oi uid yto and exhaust from said balancing chamber, and means for positioning said pilot valve including a discharge member xed. with respect 'to said -casing andhaving a discharge port controlled by the spacing with respect thereto of said control surface. f

2. A uid pressure responsive mechanism in cludnzg an enclosing casing, a movable member withinsaid casing responsive to dierential pressure, and balancing means for applying a balancing uid pressure against said movable member for balancing'the 'dierence of the pressure applied thereto, said balancing means including an expansible balancing 'chamber bounded by said movable member, a pilot valve for controlling the pressure conditions in said chambenand uid pressure responsive means for positioning said pilot valve, said last means including a discharge member having a discharge port Acontrolled by the .3. A uid pressure 'mechanism including av chamber having a iiexiblc wall and a movable i pressureV responsive closure wall, a source of iud under pressure, inlet means having. a ow restricting member therein for supplying iuid from conduit 30; when air supplied through the bub- 75' said source to said chamber, means for controlling the discharge of -iiuid from said chamber including 'a pressure control element for saidfchamber iixedly, mounted with respect to said 'chamber and having a discharge port; said closure wall directly lcontrolling the discharge through said discharge port by its positioning with respect to said discharge port.

4. A iluid pressure mechanism including a chamber having a flexible wall .and a movable pressure responsive closure wall, said closure wall having a pressure control surface. a source -of fluid under pressure, inlet means having a ilow restricting member therein for supplying iluid from said source to said chamber, means for con- .1

trolling the discharge of uid from said chamber including a pressure control element iixedly mounted with respect to said chamber and having a discharge port closely adjacent the control surface of said closure wall, the flow through said v and connected to said casing and said'rniovable` member 4to provide another static pressure chamber within said rst metallic bellows, a third metallic bellows connected to said movable member and to said casing and disposed on the opposite side of said movable member from said second metallic bellows, said second and said third metallic bellows having the same effective area with respect to said movable member whereby said static pressure chambers have equal areas with respect to said movable member, the space within said third bellows providing -a rebalancing pressure chamber for rebalancing the resultant of the pressures eiective on said movable member,

and means for controlling the pressure within said rebalancing chamber and effective on said movable member for rebalancing, said controlling means including means for supplying fluid to said rebalancing chamber through a ilow restricting member, and means controlled by the movement of said movable member for controlling the discharge from said rebalancing chamber of the supplied iluid.

6. A iluid pressure responsive unit including an enclosing casing, a movable'member within said casing and having a flat pressure control surface, means for applying a force against one side of said movable member, means for applying a force against the other side of said movable member, and additional means for applying a force against `said other side of said movable member for balancing the difference between the forces applied by said first two means, said additional means including a balancing chamber bounded in part by said movable member, means -for supplying asiaaoi for applying fluid pressure against a corresponding area on the other side of said movable member and outside said extensible wall, and balancing means for applying a balancing fluid pressure against another portion of said other side of said movable member for balancing the difference of the pressures applied by said other means, said balancing means includingA a chamber bounded in part by said movable member, means for supplying fluid to said chamber through a ilow restricting member, and means including a flxedly mounted discharge member supported by said casing and having a discharge .port controlled by the spacing of the pressure controlling surface of said movable member with respect,to said -port for controlling the discharge from said chamber of the supplied iiuid.

8. A fluid pressure unit including an enclosing casing, a movable member within said casing and 'having a flat pressure controlling surface, means forapplying a force against said movable member, and balancing means for applying a balancing tl-uid pressure against said movable member for balancing the force applied by said nrst-mentioned means, said balancing means including a chamber bounded in part by said movable member, means for supplying fluid to said chamber through a now restriction member, and means including a iixedly mounted discharge member carried by said casing and having a discharge port controlled by the spacing of said controlling surface of said movable member with respect thereto for controlling the discharge from said chamber of the supplied fluid thereby to provide within said chamber the requisite balancing pressure.

9. A fluid pressure unit including an enclosing casing, a movable member within said casing and having a. flat pressure controlling surface, means for applying forces against said movable member to provide a resultant'force, means for amplifying the effect of the resultant force applied lfluid to said chamber through a flow restricting member,.and means including a iixedly mounted against said movable member, saidA amplifyingl means including an expansible chamber bounded in part by said movable member, means for supplying fluid from a constant pressure'source to said chamber through a flow restricting'member, means controlled by the movement of said movable member for controlling the discharge from said chamber of the supplied fluid, said `'last means including a discharge member xedly mounted in' said casing and having a discharge port directly controlled `by said spacingof said controlling surface with respect thereto.

10. Pressure iiuid control mechanism including a balancing chamber bounded on one portion by a member movable in response to pressure, means for applying .fluids at different pressures against equal areas on opposite sides of said movable member outside said chamber, a source of fluid under pressure, means `for controlling the pressure conditions in said chamber for balancing including a pilot valve for controlling the sup, Dlying of fluid from said source to said chamber and the exhausting of fluid therefrom, said mov-l 

